Engines can utilize compression devices such as turbochargers and superchargers for boosting intake air. Some engines can include twin turbochargers each having a compressor arranged along separate air intake passages of the engine. Operation of the compressors can be controlled by wastegates arranged in exhaust passages bypassing the turbines. However, under some conditions, airflow provided by the separate turbochargers can become unbalanced. In particular, one of the compressors may experience surge if the turbochargers become sufficiently unbalanced.
Anti-surge valves may be used to remedy the imbalance between the turbochargers in some intake systems. However, in the example of a single anti-surge valve on a common intake system, single side compressor surge may still occur under selected conditions. For example, an engine with a common intake manifold for receiving intake air from each compressor can enter a condition where one of the compressors enters a surge condition, but the other compressor does not. Once a single side surge occurs, the surge condition can begin to oscillate between the compressors, which can cause an objectionable noise that may be referred to as noise, vibration, and harshness (NVH).
As one example, the above issues may be addressed by a method of operating a vehicle propulsion system including an engine having an intake manifold communicating with two compressors arranged in separate intake passages, the method comprising: adjusting a boost pressure provided to the engine by the first and the second compressors in response to a total flow of air consumed by the engine, said first and second compressors cooperating to provide the total flow; and during an imbalanced flow condition between the first and the second compressors, choking flow through the first compressor before the second compressor experiences surge. For example, the method may include limiting the boost pressure responsive to the total flow of air, where the choking limits a further reduction of flow through the second compressor, thereby reducing potential for surge.
In this way, a flow limitation resulting from the choke limit of a first compressor of the twin turbocharger system can be used to restrain a second compressor from entering a surge condition. For example, as the flow provided by the first compressor increases to its choke limit, any further reduction in the flow provided by the second compressor is limited by the total flow. Thus, by limiting the boost pressure to a suitable level based on a function of the total flow of intake air provided to the engine, single side compressor surge can be reduced or eliminated.
Note that there are various ways to adjust and/or limit boost pressure, including adjusting and/or limiting a corresponding compressor pressure ratio, for example.